1 Field of the Invention
The present invention relates to traveling wave optical modulators.
2 Related Art Statement
NGK Insulators, Ltd. reported in JP-A 10-133,159 that a traveling wave optical modulator was successfully operated at not less than 10 GHz by providing a thinner portion in a substrate of the modulator under an optical waveguide and setting the thickness of this thinner portion to not more than 10 xcexcm, while providing no buffer layer made of silicon oxide or the like. In this manner, the optical waveguide substrate advantageously comprises such thick and thinner portions, because optical modulation is possible at high speeds without the formation of the buffer layer, a DC drift originating from the buffer layer can be avoided, and a product (Vxcfx80xc2x7L) between a driving voltage Vxcfx80 and the length L of an electrode can be reduced.
However, NGK more specifically examined traveling wave optical modulators of this type, and then found that light absorption loss sometimes occurred in the optical waveguide. Particularly when the optical waveguides of the Mach-Zehnder type is formed, it is necessary that the optical waveguide is branched into two lines, and that lights, which come out from the branched optical waveguides are coupled together again, so that the absorption loss in each of the branched optical waveguide needs to be the same. However, the absorbing loss of one branched optical waveguide actually differed from that of the other, which might result in reduction in the extinction ratio.
It is an object of the present invention to suppress the absorption loss of the light in the optical waveguide in the traveling wave optical modulator, which can effect optical modulation at a high speed, and reduce a DC drift and a product (Vxcfx80xc2x7L) between a driving voltage Vxcfx80 and the length L of an electrode.
The present inventors succeeded in suppressing the absorption loss in a traveling wave optical modulator, by constructing the modulator from (a) an optical waveguide substrate made of an electro-optic and ferrodielectric single crystal in the form of an X- or Y-orientation plate_and comprising a thicker portion having a larger thickness and a thinner portion having a smaller thickness; (b) first and second branched optical waveguide portions formed at least on the thinner portion of the optical waveguide substrate; (c) a set of electrodes provided on at least the thinner portion of the substrate and adapted for applying voltage to the first and second optical waveguide portions to modulate a light propagating the optical waveguide portions; and (d) a buffer layer provided to cover a part of the optical waveguide portions at the thinner portion of the substrate, the electrodes crossing on the buffer layer.